In the foundry art, cores and molds used in making metal castings are generally prepared from shaped, cured mixtures of aggregate material (e.g. sand) and a binder. One of the preferred techniques of making these sand cores includes the basic steps of mixing the sand with a resin binder and a curing catalyst, molding the mixture to the desired shape and allowing it to cure and solidify at room temperature without the application of heat. Resins useful in this technique include the furfuryl alcohol-formaldehyde, furfuryl alcohol-urea-formaldehyde, and alkyd isocyanate resins as well as sodium silicate binders.
Among the resins available for this type of no-bake core making are those made by reacting phenol and aldehydes, primarily formaldehyde. A problem associated with the use of these phenol-formaldehyde resins is, however, the emission of formaldehyde fumes during the mixing of sand with the resin binder and catalyst, during the curing process, and subsequently during the casting operation, due to the presence of free or uncombined formaldehyde in the resin. This emission of formaldehyde is highly undesirable, since such odors are not only unpleasant but possibly may injure the health of workers in the area.
Ordinarily, the high viscosity of phenol-formaldehyde resins has required that they be diluted with an organic solvent such as methanol in order to obtain lower viscosities and a uniform distribution of the resin throughout the resin-sand mixture. It has been noted, however, that no-bake cores made of sand and phenol-formaldehyde resins dissolved in an alcohol solvent do not always cure satisfactorily. Instead they retain excess water, particularly in the innermost portions. The exact nature of this phenomenon is not understood, but it is thought that perhaps the migration of the more volatile alcohol vapors to near the surface of the core retards the subsequent evaporation of less volatile water from the center of the core during the curing process.